CEA, Gif-sur-Yvette
Commisariat à l'Energie Atomique (CEA), Grenoble Cedex 9
NHMFL, Tallahassee, Florida
CNR-INFM-LAMIA, Genova
Best RF bulk niobium accelerating cavities have nearly reached their ultimate limits at rf equatorial magnetic field H ~ 200 mT close to the thermodynamic critical field Hc. In 2006 Gurevich proposed to use nanoscale layers of superconducting materials with high values of Hc > HcNb for magnetic shielding of bulk niobium to increase the breakdown magnetic field inside SC RF cavities. Depositing good quality layers inside a whole cavity is rather difficult; we have made high quality samples with the techniques used for the preparation of superconducting electronics circuits and characterized these samples by PPMS, SQUID, and X-ray reflectivity. The dc magnetisation curves of 250 nm thick Nb film have been measured, with and without a magnetron sputtered coating of a single or multiple stack of 15 nm MgO and 25 nm NbN layers. The Nb samples with/without the coating clearly exhibit different behaviours. Because SQUID measurements are influenced by edge and shape effects we propose to develop a specific local magnetic measurement of HC1 based on ac third harmonic analysis in order to reveal the screening effect of multilayers.
KEYWORDS:
Particle accelerators, niobium, niobium nitride, RF cavities, thin films, superconducting multi-layers
CEA, Gif-sur-Yvette
Black Laboratories, L.L.C., Newport News
Before reaching optimal results, one needs to remove 150-200 μm by electropolishing inside niobium RF cavities, probably because the existence of a damage layer on the Nb sheet surface. Reducing the amount of electropolishing to a final light treatment would be a way to decrease both costs and risks for large projects such as ILC. We have evaluated the thickness of the damage layer after various deformations steps by observing the density of etching figures after several light chemical etches. Complementary observations with EBSD are also presented. Deep drawing brings further and deeper damage in particular in the equator region where the friction against the forming dye is the highest. Welding also influences the damage distribution. “Chemical-mechanical” polishing, a technique initially developed for the preparation of TEM samples, is a way to prepare surfaces with a very thin damage layer. It is also now applied industrially to wafer preparation and optical lenses. We think that chemical mechanical polishing of half cells before welding would be a way to decrease the thickness of the final etching for the preparation of RF Nb cavities.
CEA, Gif-sur-Yvette
DESY, Hamburg
First promising results concerning Electro-Polishing at lower voltage of 5V (abbreviated as LV-EP) has previously been reported [*,**]. This effort is being pursued and a 1-cell Tesla Shape 1.3 GHz cavity has been dedicated to LV-EP and has reached improved gradient exceeding 39MV/m. Furthermore, a second cavity has alternately been electro-polished at 5V and 17V. It did not encounter any decrease in performance after LV-EP. This process is then especially promising for the treatment of large cavities for proton applications. Moreover, long-time EP experiments on niobium flat samples show that high-voltage EP is more likely to generate impurities in the EP mixture that might contaminate cavities. Some results will also be presented concerning efficient field emission removal by chloroform rinsing of 1-cell cavities.
* F. Eozénou et al, Proc. of 13th workshop on SRF, China, (2007) TUP80
** F. Eozénou et al, CARE-Report-2008-022-SRF
KEK, Ibaraki
CEA, Gif-sur-Yvette
JLAB, Newport News, Virginia
GUAS/AS, Ibaraki
The Electro-Polishing (EP) process is the best candidate of final surface treatment for the production of ILC cavities. Nevertheless, the broad distribution of the gradient caused by field emitters in cavities is sitll a serious problem for the EP process. A candidate source of field emitter is the sulfur component which is produced in the EP process and remains the inner-surface of cavities. We studied the effect of Ethanole- and degreaser-rinse processes after the EP process by a unique method. Moreover, we tried to test the sponge cleaning as the post-EP process to remove the field emitter inside the cavcity. This article describe the results of series tests of the post-EP process at KEK.